The present invention relates to a semiconductor component with a change compensation structure and associated fabrication methods and, in particular, to a compensation component which has improved properties for interacting parameters, such as the breakdown voltage, the on resistance, and the robustness with respect to TRAPATT oscillations (Trapped Plasma Avalanche Triggered Transit).
U.S. Pat. Nos. 4,754,310 and 5,216,275 disclose theoretical investigations for semiconductor components with charge compensation structures with the object of improving the on resistance and robustness with regard to the avalanche breakdown and a short circuit in the high-current case with a high source-drain voltage.
The compensation principle of semiconductor components with charge compensation structure is essentially based on a reciprocal compensation of charges in n-doped and p-doped regions in the drift region of a transistor. In this case, the regions are spatially arranged such that the path integral over the doping along, for example, a line running vertically to the pn junction in each case remains below the material-specific breakdown charge (approximately 3xc3x971012 cmxe2x88x922 for silicon). By way of example, p-type and n-type pillars or plates or compensation regions may be arranged in pairs in a vertical transistor of the kind that is customary in power electronics.
By virtue of the extensive compensation of the p- and n-type dopings, the doping of the current-carrying region can be significantly increased in compensation components, which results in a significant gain in on resistance despite the loss of current-carrying area. In this case, the blocking capability of the transistor essentially depends on the difference between the two effective dopings. Since it is desirable for the current-carrying region to have a doping which is higher by at least one order of magnitude, for reasons of reducing the on resistance, controlling the reverse voltage requires controlled setting of the degree of compensation, which can be defined for values in the rangexe2x89xa6xc2x110%. This range becomes even smaller in the case of a higher gain of the on resistance. In this case, the degree of compensation can be defined by:
2*(p-type dopingxe2x88x92n-type doping)/(p-type doping+n-type doping)
or by
2*charge difference/charge of the doping regions.
FIG. 1 shows a simplified partial sectional view of a compensation component according to the prior art.
In accordance with FIG. 1, a plurality of semiconductor layers are situated on a carrier substrate or in a second zone 1, which is preferably n+-doped, which layers together form first compensation regions 2 of a first conductivity type (e.g. n). In this case, the charge compensation structure of the compensation component includes a plurality of compensation zones for forming a pillar-type second compensation region 3, which is connected to a source electrode or first electrode layer S via a pn-forming zone 7 or an associated zone 7xe2x80x2 of the second conductivity type (p+, p++) and a first zone 6 of the first conductivity type (n+). In this case, a drain electrode D is connected to the second zone 1. A control layer or gate electrode G formed at the surface thus realizes, together with the source and drain electrodes, a field-effect transistor. In order to insulate the gate electrode G, in accordance with FIG. 1, a first insulation layer Is1 or gate oxide layer is formed on the semiconductor material and a second insulation layer or an intermediate oxide Is2 is formed toward the source electrode.
In compensation components, in the charge compensation structure below the actual field-effect transistor, p- and n-type regions are arranged next to one another or nested in one another in such a way that, in the off-state situation, their charges can be reciprocally depleted and that, in the turned-on state, there is a non-interrupted low-resistance conduction path from the source electrode S to a drain electrode or the second electrode layer D which is connected to the second zone 1. For reasons of clarity, at this point reference is made to the description of semiconductor components with a charge compensation structure in, for example, German Patent No. DE 198 400 32 C1.
FIG. 2 shows a simplified sectional view for illustrating a fabricating step of the compensation component in accordance with FIG. 1. Identical reference symbols designating identical or corresponding layers or elements.
In accordance with FIG. 2, on an n+-doped carrier substrate or the second zone 1, a first semiconductor layer E1 and a second semiconductor layer E2 have already been deposited epitaxially for the purpose of forming the first compensation regions 2, compensation zones 4 already being situated at the interfaces between the first and second semiconductor layers E1 and E2. Accordingly, in order to realize the charge compensation structure illustrated in FIG. 1, firstly a carrier substrate 1 is provided and then a plurality of semiconductor layers E1 to Ex are formed epitaxially, for example, a plurality of compensation zones 4 being formed at the surface of the respective epitaxial layers using a respective mask 5.
Preferably, the volume to be patterned is firstly doped homogeneously with one type of charge, for example with donors (background doping). Afterward, the mask 5 is applied, for example as photoresist, and patterned in such a way that openings are produced at suitable locations. At the locations of the openings, acceptors, for example, are then introduced into the second semiconductor layer E2 for example through the use of ion implantation or conventional doping from the gas/solid phase, as a result of which firstly relatively narrowly bounded compensation zones 4 are produced at the surface. In this case, a part of this dopant concentration is not electrically active, since it is intrinsically compensated by the background doping. This part must accordingly be kept in order to obtain a desired electrically active doping.
The operation illustrated in FIG. 2 is repeated until a sufficiently thick n-multi-epitaxial layer with incorporated compensation zones 4 that are aligned with respect to one another and are stacked one above the other is present. In a subsequent step, the compensation zones 4 fabricated in this way can be outdiffused (as it were inflated) in such a way as to produce the pillar-type structure of the second compensation region 3 as illustrated in FIG. 1. This outdiffusion is preferably produced during the formation of the zones 7 and 7xe2x80x2 forming the pn junction, the first zone 6, the first insulating layer Is1 (gate oxide layer), the control electrode layer G, the second insulating layer Is2 (intermediate oxide) and also the first electrode layer S. In contrast to conventional devices or semiconductor components, the breakdown location of (power) semiconductor components with charge compensation structure in accordance with FIG. 1 does not lie near the surface, but rather preferably (for robustness reasons) deep in the voltage-accepting drift volume or semiconductor substrate E1 to Ex or 1. In the case of this construction, however, under unfavorable switching conditions extreme oscillation phenomena can arise which can destroy the semiconductor component or else the circuitry. Such a case can occur for MOS transistors with breakdown deep in the drift volume for example in the event of so-called avalanche breakdown:
In this case, firstly the MOS field-effect transistor is in the turned-on state and a very high channel current (of the order of magnitude of the rated current) is impressed via an inductance of, for example, external circuitry. If the transistor is then switched off, the inductance continues to draw current from the device, but this current is now no longer supplied as channel current, but as displacement current. As a result, the volume of the semiconductor substrate is depleted of mobile charges (space charge zone, depletion zone) and an electric voltage builds up between the first electrode (source S) and the second electrode (drain D).
As soon as the breakdown voltage is exceeded, electron/hole pairs are generated at the breakdown location and are separated on account of the high electric field and flow away in the direction of the respective electrodes S and D. An avalanche current thus arises. However, since the breakdown takes place deep in the drift volume or semiconductor substrate E1 to Ex or 1, the resulting charge packets or charge carriers L require a relative long propagation time through to the space charge boundary. Only when they have arrived there does their effect have a significant influence on the external circuitry. During this propagation time, however, the induction coil continues to draw displacement current from the device, so that the voltage present and thus also the generation rate of the avalanche charge (avalanche charges) increases more than proportionally. The voltage dips only when the charge packets arrive at the space charge boundary. From this point in time, however, an excessively high current will now flow away via the coil, resulting in a falling source/drain voltage (=negative differential resistance). On account of the decreasing voltage, however, the current generation rate decreases in the breakdown location, but this in turn does not influence the external circuitry immediately but rather only in a temporally offset manner. The above-described operation is continued analogously. A current-voltage oscillation is thus started, which is known as so-called TRAPATT oscillation, and which, under unfavorable circumstances, even begins to escalate above a predetermined threshold current. In this case, the current density can rise (locally) so high that the semiconductor component or external circuitry is destroyed.
In this case, the above-described TRAPATT mechanism (Trapped Plasma Avalanche Triggered Transit) proceeds more unfavorably, the higher the charge carrier generation in the semiconductor component. Only a simplified case has been presented in the explanatory model described above. In reality, however, in addition to the charge carriers L which arise at the breakdown location (first-order ionization), electrons and holes may also be generated at another location through impact ionization (second-order ionization).
This charge carrier generation results essentially when charge carriers L which define a current density j pass into a region where a high electric field EA prevails. This impact ionization Is is proportional to j*exp(-const./EA).
For compensation components (such as e.g. compensation modules sold under the trademark xe2x80x9cCOOLMOSxe2x80x9d) which are produced according to currently proven methods (multiple epitaxy), a situation is accordingly present wherein precisely this impact ionization or second-order ionization is formed in an extreme manner.
FIG. 3 shows a simplified graphical illustration of an electric field strength profile as a function of different distances from a center axis of the second compensation region 3. It becomes clear from FIG. 3 that the highest field strength fluctuations essentially lie in the center of a respective pillar or a second compensation region 3. In this case, FIG. 3 essentially represents a depth-resolved profile of an electric field at various sectional positions within a cell for a xe2x80x9cCOOLMOSxe2x80x9d product, the number of layers differing from the structure illustrated in FIG. 1.
As is evident from FIG. 3, the highest field strengths of the semiconductor component lie in or near the center of the p-type pillar (n-type pillar) or of the second compensation region 3. It is precisely there, however, that the holes (electrons) L generated in the breakdown are focused, as it were, if they flow away from a deep breakdown location for example in the direction of the first electrode S (source). In this case, the equipotential lines act like a converging lens for the charge carriers L, whereby splitting, as it were, is generated. Since very high current densities are thus accompanied by very high field maxima in the center of the pillar or of the second compensation region 3, the above-described secondary ionization supplies a substantial proportion of the entire avalanche breakdown current, as a result of which the TRAPATT oscillation is amplified or even caused in the first place.
In order to increase the robustness with respect to TRAPATT oscillations, field strength peaks (primarily in a charge carrier path prescribed in the breakdown situation) must be reduced (e.g. through changes in the depth-resolved charge balance of the drift volume). However, this generally reduces the breakdown voltage. This effect can be lessened or eliminated by greater outdiffusion of the compensation zones 4 (e.g. FIG. 2). As a result, however, the n-conducting current path is then constricted again, that is to say the on resistance is impaired and the TRAPATT susceptibility is slightly increased. This cause-and-effect chain shows that the set of parameters (breakdown voltage, robustness with respect to TRAPATT oscillations and on resistance) is closely coupled to one another. Improving one of these parameters generally impairs at least one of the other two parameters with previously known methods.
It is accordingly an object of the invention to provide a semiconductor component with a charge compensation structure and associated fabrication methods which overcome the above-mentioned disadvantages of the heretofore-known semiconductor components and fabrication methods of this general type and which provide improved electrical properties. In particular, the invention is intended to reliably achieve improvement of one parameter, such as e.g. a high breakdown voltage, without, as usual, realizing this improvement at the expense of other parameters such as e.g. an optimized on resistance and/or a high robustness with respect to TRAPATT oscillations.
With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor component, including:
a semiconductor substrate;
a first electrode layer;
a second electrode layer;
a charge compensation structure having a blocking pn junction in the semiconductor substrate, a first zone of a first conductivity type connected to the first electrode layer, a zone of a second conductivity type opposite to the first conductivity type connected to the first electrode layer and adjoining the first zone of the first conductivity type for forming a blocking pn junction, a second zone of the first conductivity type connected to the second electrode layer, a first compensation region of the first conductivity type and a second compensation region of the second conductivity type nested into one another in a region between the first zone of the first conductivity type and the second zone of the first conductivity type;
the first compensation region and the second compensation region forming a pn junction therebetween, the pn junction between the first compensation region and the second compensation region defining a compensation edge region; and
the first compensation region, the second compensation region and the compensation edge region being configured such that locations with a maximum local field strength are situated in the compensation edge region.
In other words, a semiconductor component with a charge compensation structure, which has a blocking pn junction in a semiconductor substrate, having a first zone of a first conductivity type, which is connected to a first electrode layer and adjoins a zonexe2x80x94forming the blocking pn junction of a second conductivity type opposite to the first conductivity type, which is likewise connected to the first electrode layer, and having a second zone of the first conductivity type, which is connected to a second electrode layer, a first compensation region of the first conductivity type and a second compensation region of the second conductivity type being nested or interleaved in one another in the region between the first zone and the second zone, and the pn junction between the first and second compensation regions defining a compensation edge region, wherein locations with maximum local field strength are located in the compensation edge region.
In particular by forming a doping such that locations with locally maximum field strength are displaced into a compensation edge region, it is possible to substantially improve one parameter, e.g. the on resistance of the compensation component, without influencing or impairing the other parameters, e.g. breakdown voltage and robustness with respect to TRAPATT oscillations.
In particular by setting a ratio V=Etr,max/Ev,max greater than 1, where Etr represents a lateral and Ev a vertical field strength and xe2x80x98maxxe2x80x99 in each case denotes a corresponding field strength peak, it is possible to shift the locations of maximum field strengths (field strength peaks) completely into the edge region.
According to another feature of the invention, the first and second compensation regions haven increased dopant concentrations selected such that a lateral field strength is increased without changing a charge compensation capability.
In order to increase the lateral field strength, it is possible, for example, to significantly increase a dopant concentration in the first and second compensation regions (for example the increase may amount to approximately 10% compared with known devices), i.e. usually approximately 1xc3x971014 to 10xc3x971016 cmxe2x88x923, as a result of which, in an advantageous manner, the resistance of the conducting path is reduced without influencing the breakdown voltage or the TRAPATT susceptibility.
According to another feature of the invention, the first and second compensation regions have an reduced doping ripple selected such that the doping ripple is reduced in order to reduce a vertical field strength.
According to another feature of the invention, the second compensation region has an increased number of compensation zones in the vertical direction for reducing the doping ripple.
According to another feature of the invention, the semiconductor substrate has an increased number of epitaxial layers for reducing the doping ripple.
According to another feature of the invention, the charge compensation structure has a hexagonal shape or a strip shape when viewed in a plan view.
According to another feature of the invention, the semiconductor component is a compensation component.
In order to reduce a vertical field strength, it is possible to reduce a ripple of the first and second compensation regions, thereby again producing improved TRAPATT properties. A reduced doping ripple can be realized for example through the use of an increased number of compensation zones and, in particular, through the use of an increased number of epitaxial layers in the semiconductor substrate.
With the objects of the invention in view there is also provided, a method for fabricating a semiconductor component according to the invention, the method includes the steps of:
providing a carrier substrate;
forming a plurality of semiconductor layers in order to realize the first compensation region;
carrying out a masked doping at least in a part of the semiconductor layers in order to form compensation zones and in order to realize the second compensation region nested in the first compensation region;,
carrying out an unmasked doping with a conductivity type opposite to a conductivity type of the masked doping, in at least one of the semiconductor layers, in order to shift the locations with the maximum local field strength into the compensation edge region; and
forming elements and layers for driving the first and second compensation regions.
A preferred mode of the method according to the invention includes introducing boron as a dopant for the masked doping and introducing phosphorus as a dopant for the unmasked doping.
With the objects of the invention in view there is also provided, a method for fabricating a semiconductor component according to the invention, the method includes the steps of:
providing a carrier substrate of the first conductivity type in order to realize the first compensation region;
forming a trench in the carrier substrate;
filling the trench with a semiconductor material of the second conductivity type in order to form the second compensation region; and
forming elements and layers which drive the first and second compensation regions.
A preferred mode of the method according to the invention includes forming the trench by using an anisotropic etching method.
With regard to the fabrication method, a semiconductor component with improved TRAPATT properties can be fabricated in a particularly simple and cost-effective manner in particular by formation of a plurality of semiconductor layers with a respective masked doping in order to form compensation zones or in order to realize the second compensation regions and at least one unmasked doping of the opposite conductivity type in order to displace the breakdown locations into the compensation edge region.
Preferably, for the masked and the unmasked doping, dopants of the opposite conductivity type that diffuse essentially equally rapidly are introduced through the use of ion implantation, as a result of which the fabrication method can be simplified further. A respective implantation energy is selected in such a way that the implantation maxima of the introduced dopants essentially lie at the same depth (in which case, of course, even in the event of additional whole-area implantation, the compensation conditions must be preserved, that is to say that the corresponding charge must be taken into account, i.e. must be kept in the masked implantation).
As an alternative, however, it is also possible to form a trench in a carrier substrate and to fill the trench with a semiconductor material of the opposite conductivity type to the carrier substrate, as a result of which a doping ripple can be completely eliminated and, accordingly, the breakdown locations are located precisely at the pn junction between the first and second compensation regions. In the case of a semiconductor component fabricated according to a method of this type, essentially a symmetrization of a hole path to an electron path is obtained, as a result of which the TRAPATT properties are improved further.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a semiconductor component with charge compensation structure and associated fabrication methods, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.